1. Field of the Invention
The present invention relates to the field of mass spectrometry and more specifically to an ion guide and its method of use.
2. Discussion of Related Art
Ion guides are well known in the mass spectrometry art for the efficient transport of ions between regions of successively reduced pressure. The ion guide generally includes a plurality of electrode pairs arranged symmetrically about the central longitudinal ion flow axis. An oscillating radio frequency (RF) voltage is applied in a prescribed phase relationship to the electrode pairs to generate a multipole field that confines ions to the interior of the ion guide. While quadrupole ion guides (consisting of two electrode pairs to which opposite phases of the RF voltage are applied) are most commonly employed in mass spectrometers, multipole ion guides utilizing a greater number of electrode pairs and generating higher-order fields (e.g., hexapole or octopole) are also known.
The electrodes of prior art multipole ion guides generally take the form of conductive rod electrodes, having a substantially invariant lateral cross-section, elongated along the central ion flow axis. Typically, the rod electrodes have a cylindrical shape with a circular lateral cross-section. It is also known to use rod electrodes having a square lateral cross-section, although such electrodes generate a greater degree of higher-order electric fields, which may have an adverse effect on transmission efficiencies. In order to produce a “purer” quadrupolar field, it is known to use rod electrodes having a complex cross-section with a hyperbolic inner facing surface, but hyperbolic rod electrodes are difficult and expensive to manufacture, and so their use is typically limited to devices, such as linear ion traps and quadrupole mass filters, in which control and characterization of the generated field is critical.
One problem exhibited by prior art ion guides is the occurrence of field breakdown, causing arcing (spark discharge) between adjacent electrodes. The rod electrodes terminate at their ends in flat faces, thereby defining sharp corners that facilitate arcing. Arcing, which may result in substantial damage to the electronics and data system, may be particularly problematic when the ion guide is positioned within a region of relatively high pressure and/or when fields of relatively great magnitude are employed.